The NOP receptor, previously called the opioid receptor-like receptor (ORL1, XOR1 and LC132) belongs to the opioid receptor family and has nucleotide and amino acid homology to the mu, delta and kappa opioid receptors. However, the NOP receptor does not bind opiate ligands with high affinity, as would be expected for an opioid receptor. The endogenous 17-amino acid peptide ligand for NOP, nociceptin or orphanin FQ (N/OFQ), poor affinity for mu, delta, and kappa opioid receptors.
N/OFQ, when injected intracerebroventricularly (i.c.v. or ICV) into mice, leads to a decrease in hot plate escape jumping latency and a decrease in tail flick latency due to attenuation of stress-induced, opioid-mediated antinociception. Additional studies have demonstrated the presence of NOP and N/OFQ precursor protein and mRNA in pain-processing pathways.
A growing body of evidence suggests that the N/OFQ-NOP receptor system plays a significant role in the reward process and drug abuse. There is a moderate to high density of NOP receptors in areas implicated in drug reward, including the nucleus accumbens, ventral tegmental area, medial prefrontal cortex, lateral hypothalamus, amygdala and the bed nucleus of stria terminalis. ICV administration of N/OFQ suppresses basal and drug-stimulated dopamine release in the nucleus accumbens. N/OFQ has been shown to block the rewarding properties of several common drugs of abuse. In particular, N/OFQ can block acquisition of conditioned place preference (CPP) induced by morphine, cocaine, amphetamines and alcohol.
The inhibitory effect of N/OFQ on morphine CPP and the inhibition of morphine-induced dopamine release in mesolimbic areas suggest that N/OFQ may function as an “anti-opioid” peptide with respect to reward as well as pain (Ciccocioppo, R., et al., Peptides, 2000, 21(7): 1071-1080). These studies confirmed the involvement of NOP receptors in drug addiction and suggested the utility of a NOP agonist as a drug abuse medication
Every year, about 100 million adult Americans experience some form of pain, a condition that costs the nation between $560 billion and $635 billion annually in lost productivity and treatment (“Relieving Pain in America: A Blueprint for Transforming Prevention, Care, Education and Research; Institute of Medicine of the National Academies, June 2011). Opioid analgesics are the mainstay of pain treatment and often the only treatment option that provides significant relief. However, opioid analgesics (which are mainly mu opioid receptor (MOP) agonists) are controlled substances that have abuse potential and are riddled with many lifestyle side effects such as constipation, nausea and tolerance, that impede their long-term safety and effectiveness and create other societal disorders (the abuse of prescription pain-killers). Analgesics that do not have opioid-related liabilities are therefore vital for addressing the large need for safe and effective pain treatments, as mandated in the recently released National Pain Strategy (NINDS, Interagency Pain Research Coordinating Committee. National Pain Strategy; NIH NINDS: 2015. http://iprcc.nih.gov/National_Pain_Strategy/NPS_Main.htm).
From the opioid receptor family of mu, delta (DOP), kappa (KOP) and nociceptin (NOP) opioid receptors, KOP and DOP agonists have also been investigated as analgesics but do not show strong analgesia compared to MOP agonists, and show poor dose-separation with compromising side-effects such as dysphoria (for KOP agonists) (Wadenberg CNS Drug Rev., 2003, 9(2): 187-198) and convulsions (for DOP agonists) (Negus et al., J. Pharmacol. Exp. Ther., 1994, 270(3): 1025-1034; Negus et al., J. Pharmacol. Exp. Ther., 1998, 286(1): 362-375). Kappa-type agonist-antagonists such as nalbuphine, butorphanol have been used clinically for decades, but are considered weaker analgesics than MOP-based analgesics.
On the other hand, NOP receptor-targeted ligands are clearly emerging as potential analgesics, from recent developments (Lin et al., ACS Chem. Neurosci., 2013, 4(2): 214-224; Linz et al., J. Pharm. Exp. Ther., 2014, 349(3): 535-548; Lambert et al., Br. J. Anaesthesia, 2015, 114(3): 364-366). The NOP receptor and its endogenous ligand N/OFQ are the fourth members of the opioid family. The NOP receptor is present in the same pain pathways as the other opioid receptors and has a generally inhibitory function on neuronal transmission. The role of the NOP receptor in pain and analgesia is revealing new data, that suggests that NOP agonists may have superior analgesic potential, similar to mu opioid agonists like morphine, but not possess other liabilities like dependence and respiratory depression (Podlesnik et al., Psychopharmacology, 2011, 213(1):53-60; Sukhtankar et al., Res. Dev. of Opioid-Related Ligands ACS, 2013, 1131:393-416).
Studies with systemic administration of nonpeptide NOP agonists reveal that NOP agonists have potent anti-nociceptive activity in several animal models of pain, particularly neuropathic and inflammatory pain (Khroyan et al., Eur. J. Pharmacol., 2009, 610(1-3):49-54; Khroyan et al., J. Pharmacol. Exp. Ther., 2011, 339(2):687-693; Sukhtankar et al., Psychopharmacology, 2014, 231(7):1377-1387). Notably, studies in non-human primates have been more encouraging and consistent, compared to studies in rodents and show that peptide NOP agonists such as N/OFQ and UFP-112 produce spinal antinociception in primates, when administered intrathecally (Hu et al., Pain, 2010, 148(1):107-113) and nonpeptide NOP agonist Ro64-6198 given s.c. produces antinociception against capsaicin-induced allodynia and thermal pain (Podlesnik, et al., Psychopharmacology, 2011, 213(1): 53-60). Antinociceptive potencies and efficacy of NOP agonists was comparable to that of morphine (Sukhtankar et al., Psychopharmacology, 2014, 231(7):1377-1387), but importantly, there was an absence of itch, respiratory depression and reinforcing effects at efficacy doses. These findings in primates strongly support the clinical potential of NOP agonists as a novel approach for “analgesia without opioid liabilities” (Lin et al., ACS Chem. Neurosci., 2013, 4(2): 214-224).
Studies have shown that bifunctional NOP/mu opioid agonists may also provide a novel approach for developing non-addicting analgesics (Khroyan et al., J. Pharmacol. Exp. Ther., 2007, 320(2): 934-943; Khroyan et al., J. Pharmacol. Exp. Ther., 2011, 339(2):687-693). Others have further confirmed that nonpeptide bifunctional NOP/mu agonists show potent antinociceptive effects in rodent and primate models of pain (Linz et al., J. Pharm. Exp. Ther., 2014, 349(3): 535-548).
Parkinson's disease (PD) is clinically characterized by hypo/akinesia, rigidity, gait disturbance and resting tremor and other non-motor symptoms such as depression and cognitive decline. PD is a costly disease both for the individual and society. Annually, the PD patient spends significantly more than healthy individuals in direct costs annually (e.g. drugs and hospitalization) and indirect costs (e.g. absence from work, early retirement; informal home care). Therefore, in the economics of PD treatment, therapies that forestall motor disabilities or cognitive impairment would provide a significant reduction in indirect costs with minimal increases in overall medication costs. It has been long recognized that PD patients stably responding to dopaminergic therapy (the current first choice for therapy) gradually develop two progressive clinical phenomena, motor fluctuations and dyskinesias (involuntary movements), that are even more disabling, and for which there is “only one” FDA-approved therapy. The dopamine (DA) precursor levodopa (L,3,4-dihydroxyphenylalanine; L-DOPA) is the cornerstone of PD therapy, often given currently in combination with COMT and MAO inhibitors, to extend its bioavailability and therapeutic action.
However, chronic L-DOPA therapy is associated with the eventual appearance (within 10 years in ˜80% of patients) of motor complications (motor fluctuations and dyskinesias) that limit its clinical effectiveness and greatly reduce the quality of life of patients. Thus, the development of drugs able to delay the development of dyskinesia and/or to attenuate its expression in already dyskinetic patients is the main unmet medical need in PD. Levodopa-induced dyskinesias (LID) represent an important cause of disability and social distress in PD patients, contributing to the risk of falls and the necessity for caregivers, especially in advanced PD cases that have other neurodegenerative pathologies (i.e., memory complaints, hallucinations and comorbidities) (Schrag et al., Mov. Disorders, 2007, 22:938-945. There are very limited treatment options for dyskinesia and the only marketed antidyskinetic drug, amantadine, a glutamate antagonist, has poor and short-lasting clinical efficacy.
The N/OFQ-NOP receptor system is widely expressed in brain cortical and subcortical areas, particularly in striatum, globus pallidus and substantia nigra (SN) neurons, areas that undergo neurodegenerative changes in PD. Endogenous N/OFQ has been shown to contribute to development of PD symptoms because N/OFQ levels are elevated in the SNr following dopamine (DA) cell loss or impairment of DA transmission. Such an increase is also observed in the CSF of PD patients (Marti, et al., 2010). NOP receptor antagonists reverse parkinsonian symptoms in neurodegenerative (6-OHDA hemi-lesioned rat, MPTP-treated mouse and macaque) and functional (reserpinized or haloperidol-treated animals) models of PD. Genetic deletion of the N/OFQ gene protects mice from the neurotoxic action of MPTP. Mechanistic studies revealed that the antiparkinsonian action of NOP antagonists is accomplished through normalization of the imbalance between excitatory (GLU) and inhibitory (GABA) inputs impinging on nigro-thalamic neurons, generated by striatal DA deafferentation. NOP antagonists also potentiate the symptomatic effect of levodopa. Therefore, NOP receptor antagonists may provide symptomatic and neuroprotective benefit in PD patients.
On the other hand, NOP receptor agonists have been shown to attenuate the expression of abnormal involuntary movements (AIMs, a rodent correlate of LID) in dyskinetic rats and nonhuman primates challenged with L-DOPA. Thus, NOP receptor ligands have promising efficacy in Parkinson's disease animal models.
Ito et al., International Publication No. WO 2005/016913 and Spear et al., International Publication No. WO 2014/106238 disclose compounds with activity at the NOP receptor with utility as treatments for pain and CNS disorders. Piperidinyl-containing compounds with activity at the NOP receptor have been disclosed in Zaveri et al., U.S. Patent Application No. 2005/0228023, Tafesse, U.S. Patent Application No. 2015/0315201 and Mustazza et al., J. Med. Chem. 2008, 51:1058-1062. Allen et al., U.S. Patent Application No. 2013/0225552 discloses heterobicyclic compounds that are PDE10 inhibitors. However, there is still a need for novel NOP receptor ligands.